Over the past decade, the demand for increased speed (gigabit communications) has forced the data communications infrastructure to evolve beyond the limits of traditional copper based transmission media to the higher bandwidth achievable with fiber optics. The emergence of these high bandwidth communication systems has created a need for standards-based optical transceiver components. While edge-emitting lasers remain the dominant technique for long-wavelength Fabry-Perot type transceivers, vertical cavity surface emitting laser (VCSEL) technology has become more cost effective for manufacturing short-wavelength transceivers and are emerging for long wavelength applications. In addition single mode lasers have advantages in terms of high speed data transmission due to the well define rise and fall time of the single optical mode. When simulating emissions from lasers with multiple modes complex interactions between the modes and the carrier lead to non-deterministic results that are typified in digital systems as jitter. Reducing jitter is one of the challenges to increasing data rates from single Gigabit/sec rates to tens of Gigabits/sec and above. Another advantage of single mode lasers is the pure spectral content, beam shape and high coherency which is desirable for sensor application such as for example, laser scanners, interferometers, position sensors and medical spectroscopic sensors.
A principal characteristic of a VCSEL is that it emits beams vertically, i.e. in a direction normal to the p-junction of the semiconductor wafer from which it was fabricated. Historically, VCSELs have been fabricated using crystalline growth techniques to deposit many layers of semiconductor material upon a substrate. These lasers include highly reflective surfaces above and below an active layer, forming a short laser cavity perpendicular to the active layer plane. This enables laser output normal to the surface of the device instead of horizontally, as with edge-emitting lasers. Vertical cavity devices thus take up less space and require less power to drive the lasing action.
VCSELs generate light by a laser current flowing through a relatively large cross-sectional area of semiconductor material in a light-generating region. When the laser current is just above the threshold level, a VCSEL operates in a single transverse -mode. However, when the laser current is increased beyond a second, higher, threshold level, the laser begins to emit a light beam with a multi-mode distribution. Known VCSELs typically use a short optical cavity which inherently limits operation to a single longitudinal (or axial) mode. However, VCSELs are typically on the order of about three microns versus twenty microns across so that lateral (or transverse) modes tend to become multi-mode due to the large transverse dimensions relative to the lasing wavelength. VCSELs sized to ensure single-mode operation suffer from increased thermal impedance and electrical resistance which restricts the VCSEL""s range of operation.
Many approaches for controlling the transverse mode structure have been developed in recent years. One approach controls the current injection profile to match the desired mode. For example, U.S. Pat. No. 5,245,622 (hereinafter known as the ""622 patent) which issued to Jewell et al on Sep. 14, 1993, describes a vertical cavity surface emitting laser with intra-cavity structures. The intra-cavity structures allow the vertical cavity surface emitting laser to achieve low series resistance, high power efficiencies and TEM00 mode radiation. In one embodiment of the invention, a VCSEL comprises a laser cavity disposed between an upper and a lower mirror. The laser cavity comprises upper and lower spacer layers sandwiching an active region. A stratified electrode for conducting electrical current to the active region is disposed between the upper mirror and the upper spacer. The stratified electrode comprises a plurality of alternating high and low doped layers for achieving low series resistance without increasing the optical absorption.
The VCSEL disclosed in the ""622 patent further comprises a disc shaped current aperture formed in the stratified electrode by reducing the conductivity of an annular region of material surrounding the current aperture. The current aperture suppresses radiation of higher mode. In another embodiment, a metal contact layer having an optical aperture is formed within the upper mirror of the VCSEL. The optical aperture blocks the optical field in such a manner that introduces lasing of higher order transverse modes. The content of the ""622 patent is hereby incorporated by reference as if set forth in fill.
Alternative approaches to single mode laser operation spatially modulate the optical loss so as to increase the threshold of selected modes. The principle of spatial mode control is shown below in FIG. 1. The optical loss, given by L=1xe2x88x92R where R is the reflectivity as seen from the laser cavity, is modulated spatially. The overlap integral of the intensity profile Im(r) with the optical loss L(r) makes the modal loss Lm as given in Equation 1. By making the modal loss significantly higher for the undesired modal profiles, those modes may be selected to be suppressed by design.
xe2x80x83Lm=∫∫Im(r)L(r)r dr dxcex8xe2x80x83xe2x80x83(1)
An example of spatial mode control is disclosed in U.S. Pat. No. 5,940,422 (hereinafter known as the ""422 patent) which issued to Johnson et al on Aug. 17, 1999. The ""422 patent discloses a vertical cavity surface emitting laser having a mode control structure that selectively encourages or inhibits the lasing of the laser in regions of the mode control structure. The mode control structure is a patterned dielectric coating on the lasers surface which is stepped from an optical thickness of xc2xd xcexs (where xcexs is the wavelength in the material) in the regions where lasing is desired to xc2xc xcexs in regions where lasing is not desired. The content of the ""422 patent is hereby incorporated by reference as if set forth in full.
A further example of spatial mode control is disclosed in U.S. Pat. No. 5,753,941 (hereinafter known as the ""941 patent) which issued to Shin et al on May 19, 1998. The Shin patent discloses a vertical cavity surface emitting laser with an electrode layer comprising a metal layer having a high electrical conductivity and a conductive auxiliary reflector layer formed between the metal layer and the VCSEL semiconductor mirrors. The auxiliary reflector layer is formed of nickel, molybdenum, platinum or chromium. The reflectivity of the auxiliary reflector layer is approximately 98-99%, which is lower than the reflectivity of the semiconductor mirrors. Therefore, the intensity of the light reflected from the lower surface of a protruding portion of the auxiliary reflector layer is less than the intensity of the light reflected from the surface without the metal. Thus, the auxiliary reflector layer suppresses the emission of light of higher order modes and thus serves to emit low-noise light of a single mode via the cavity. The content of the ""941 patent is hereby incorporated by reference as if set forth in full.
An additional example of spatial mode control is disclosed in a PhD thesis entitled xe2x80x9cVertical-Cavity Surface-Emitting Lasers: Tailoring of Optical Admittances,xe2x80x9d by Kevin J. Knopp, University of Colorado at Boulder, 1999 (hereinafter the Knopp reference) which describes the use of an anti-phase metal on the surface of the upper semiconductor mirror to introduce spatially selective loss. The loss is increased at the periphery of the laser, thereby effecting the higher order modes and preferentially selecting the fundamental mode. Equations for the reflectivity of the semiconductor mirrors with either air or metal terminations are presented and convoluted with the modal intensity profiles to provide estimates of the modal loss discrimination. The Knopp reference is hereby explicitly incorporated by reference.
However, conventional methods for controlling the operating mode of a laser are either complicated to manufacture or utilize only one of the two major approaches to mode control. Some provide for discrimination between the threshold of different modes by introducing spatially varying losses while others adjust the current injection to match the spatial profile of the desired modes. For example, the ""622 patent requires a relatively complicated fabrication sequence and only addresses spatial matching of the current to the fundamental mode. Similarly, the ""422 patent teaches the use of optical loss modulation via anti-phased dielectrics but does not teach or suggest the benefits of spatial current modulation. Therefore, the non-lasing regions will be electrically pumped but will not lase, leading to higher overall threshold currents and slower optical rise and fall times as carriers from the non-lasing regions of the active layer diffuse into the lasing regions. Similarly, the ""941 patent integrates a low reflectance metal under the ohmic contacts to realize spatially varying losses. However, the low reflectance metal may make the device resistance difficult to control. In addition, the ""941 patent does not utilize spatial current modulation.
The Knopp reference suggests a single mode laser design using anti-phased gold, silver or aluminum metal annulus. However, the anti-phase resonance achieved by the Knopp technique has a very narrow spectral band, making the manufacturing difficult. In addition, no method of transverse index guiding to control the lateral profile of the modes is taught. Further, no spatial current modulation, or current constriction is taught or discussed.
Therefore, it would be advantageous to provide a system and a robust method of providing spatial variation in current and optical loss that may be readily implemented, to provide a highly manufacturable single-mode VCSEL.